1. Field of the Invention
This invention relates to the method of simultaneous grinding of coal and dolomite in preparation for injecting the mix into a combustion chamber, and apparatus for accomplishing the method.
2. Description of the Prior Art
The development of atmospheric fluidized bed combustion requires an absorbent for the sulphur dioxide, such as the addition of dolomite to the coal to minimize sulphur emissions, as the dolomite reacts with the sulphur in the coal during combustion. Dolomite is a mineral of carbonate of calcium and magnesium, and when mixed with coal in a combustion environment chemically reacts with the sulphur in coal to greatly reduce the sulphur emission. In the grinding of dolomite it fractures and produces a large amount of fine particles of the order of minus 140 mesh when grinding is set for producing minus one-eighth size which is 6 mesh.
The use of coal in atmospheric fluidized bed combustion needs to be carried out with control over the sulphur dioxide emission. This control is accomplished with dolomite to produce the calcium to sulphur (CA:S) mole ratio for removal of a high percentage of sulphur dioxide. Particle size is important to utilize the absorption reaction from dolomite. If the particles are too fine the fluidizing thereof will carry them through the bed. The problem is to get a proper particle size when grinding the dolomite.
Since coal and dolomite grind differently, it has been the practice to grind each separately and to thereafter mix them in the desired proportions. If the setting for grinding coal is used for grinding dolomite it results in shattering the dolomite into a too fine state, and when coal is processed at a dolomite setting it is too coarse. As an example, the crushing hardness and density of coal and dolomite varies greatly, as for example: coal grinding yields about 99.8% particles of 6 mesh, 13.3% minus 140 mesh, a density of 45.6 pounds per cubic foot, at about 0.1% surface moisture. The dolomite yields about 97.8% particles of 6 mesh, 20.1% minus 140 mesh, a density of 109.7 pounds per cubic foot, at about 0.3% surface moisture, but at a substantially different air setting.
The separate grinding circuits include a separate crushing system for coal and dolomite. It also requires separate collecting bins, and a mixing circuit with necessary conveyors and a separate bin for the mixed product.
An example of grinding coarse and fine material in a common mill is found in Hardinge U.S. Pat. No. 2,381,351 of Aug. 7, 1945, the system of this disclosure being limited to separating a single type of material into coarse and fine portions and controlling the quantity of each portion allowed to enter the mill where the collecting of the coarse material is used to effect the grinding and crushing of the total content of the mill.
The presence of particulate grindings of minus 140 mesh presents problems in pressurized fluid bed combustors. The injectors used to inject the mixture of two parts of coal and one part of dolomite have a tendency to plug with the excess minus 140 mesh particulate matter. After injection into the pressurized fluid bed combustor the minus 140 mesh particulate matter has a tendency to pass directly through the combustion zone resulting in a loss of carbon. It has already been pointed out above that there is the additional problem of drying, grinding, sizing, storing, and mixing the dried coal and dolomite when prepared as separate products prior to injection.